Existing digital transmission equipment using telephone pairs requires the removal of bridged-taps on the pair to be used if the taps exceed a certain individual or cumulative length. The allowed length of tap depends on the transmission bit-rate and the velocity factor of the cable used, and is chosen to make the pulse distortion due to echoes small compared to both the height and width of the pulses themselves, and the distortion introduced by the cable pair minus taps. Unfortunately, the Serving Area Concept now used for providing telephone loops virtually guarantees that bridged-taps will be present on typical lines. Removal of these taps from all lines used for providing digital service will become increasingly impractical as the number of subscribers requiring digital service continues to grow.
Automatic equalizers (or Automatic Line Build Out Networks - ALBO) which work properly on pairs without bridged-taps do exist. These are widely used on T-carrier lines which carry continuous, rather than bursted, streams of pulses. ALBO's work by detecting the average pulse height using a rectifier and filter arrangement and then using the detected height to adjust the gain of an AGC amplifier and the frequency response of a controllable equalization network. The gain and frequency-response adjusted pulses are then applied to the data detectors, usually consisting of one or more comparators. This method requires an initial knowledge of the relationship between received pulse height and equalization settings. This knowledge is designed into the ALBO initially and therefore limits the ALBO to use on pairs which have a loss vs. frequency response characteristic close to the one assumed. Any frequency domain filtering used to control interference at either the sender or receiver must be explicitly accounted for in the ALBO design. Any change in the transmitter output level causes equalization errors at the receiver, which interprets level changes as reflecting only loop losses. Finally, since reflections from bridged-taps and gauge changes produce frequency-domain changes which are both more complex than those caused by the loop itself and not sensibly related to the loss which they cause, ALBO type arrangements are inherently incapable of correcting pulse distortion from these causes.
A further difficulty with existing equalizers results from the methods used to extract timing from the incoming data pulses. In T-carrier systems, where a continuous pulse stream is available, the zero-crossings of either the equalized or unequalized pulses are used to shock-excite a resonator (i.e., a tuned circuit, quartz crystal, etc.) which rings at its resonant frequency, equal to the intended pulse rate or a multiple. The resulting "ringing" of the resonator produces the clock signal for the data recovery. Due to the limited resonator "Q," this method requires a pulse stream with some minimum density of zero crossings and no lengthy gaps. It is therefore unsuitable for "ping-pong" or TCM use because the gaps in the data required to implement full-duplex transmission are too long to maintain "ringing" in the resonator. An additional problem is that equalization errors or echoes can cause jitter and long-term errors in the clock phase, resulting in non-optimum sampling of the pulses.
Another method of timing recovery uses a free-running crystal-controlled clock at some multiple of the bit rate. When zero-crossings occur in the data, a determination is made as to whether the zero-crossing was early or late and the clock phase is adjusted in the same direction. This method permits bridging of gaps between bursts of pulses, but still allows timing errors due to equalization errors or echoes.
One attempt to overcome the limitations of ALBO type equalization is called a "Decision Feedback Equalizer". In this method it is assumed that the received bit pattern is approximately right, and for each possible combination of previous bits (for example the 3 bits just past) a record is kept of whether the zero-crossing immediately following was early or late. The early or late status of the zero-crossing is assumed to carry information about whether the immediately preceding bit was rendered higher or lower in amplitude than it should have been by the cumulative effect of the preceding bits. Alternatively, the height of each bit could be measured directly and correlations developed from these measurements. The results of the correlations are used as a correction to each bit so that the threshold level used for detection of a given bit depends on the preceding bits. Unfortunately, this method of equalization does little to correct timing jitter induced by equalization errors. It is also subject to timing and equalization errors due to patterns in the data bits. Worst of all, it creates a feedback loop in which the equalization parameters are derived from the bits being equalized, causing potential stability problems.